This invention relates to a method for performing nuclear magnetic resonance (NMR) studies. More specifically, this invention relates to an NMR method useful for performing NMR studies in a shorter time than with conventional schemes, but with substantially the same reduction in baseline error.
U.S. Pat. No. 3,781,650 discloses a method for reducing interference in the receiver of a spin resonance spectrometer. In accordance with the method, a first set of radio frequency (RF) excitation pulses of one phase angle, and a second set of RF excitation pulses which are 180.degree. out of phase with the first set are generated. The free precession decay signals produced by one set of pulses is subtracted from the free precession decay signals produced by the other set. The signals from the excited sample reinforce, while the interference signals cancel.
The method is disclosed in the above-identified Patent with respect to an NMR analytical spectrometer and, therefore, is not concerned with scan-time reduction and magnetic field gradients effects, as is the case in NMR studies of human subjects. Scan-time reduction is important in the study of human subjects to increase patient throughput in the case of medical application of NMR and to decrease the likelihood of patentmotion artifacts. Magnetic field gradients are needed in NMR studies, such as imaging, to encode spatial information into the NMR signal to enable image reconstruction.
U.S. Pat. No. 4,443,760, assigned to the same assignee as the present invention and which is incorporated herein by reference, discloses and claims a method for eliminating interference from spurious free induction decay (FID) signals created by imperfect 180.degree. RF time-reversal pulses utilized to produce NMR spin-echo signals. One way this is accomplished is to phase shift by 180.degree. relative to one another successive excitation pulses and subtracting the resulting NMR signals thereby eliminating the spurious FID signals. This method is also effective in eliminating other error components such as d.c. voltage offsets and residual effects associated with magnetic field gradients. The spurious FID signals, d.c. voltage offsets and residual effects of gradients will be hereinafter collectively referred to as "baseline error component." Another way to eliminate the effects of spurious FID is to phase shift successive spin-echo-producing 180.degree. RF pulses and to add the NMR signals. The spurious FID signals are phase shifted and cancel, while the desired signals reinforce.
A drawback associated with the use of phase-alternated RF excitation of the type described above with reference to U.S. Pat. No. 4,443,760, is that in NMR imaging applications portions of the imaging pulse sequence are repeated with the sign of the 90.degree. RF pulse reversed. In some applications, this may be desirable, since not only is the noise removed, but the signal-to-noise ratio is improved. In some systems, notably those utilizing high magnetic fields, the signal-to-noise ratio may not require enhancement. In this case, the method results in unnecessarily lengthening the data collection process.
Accordingly, it is an object of the present invention to provide a method for substantially eliminating the baseline error component, while shortening the data collection time.